As this type of a tool path generation method, in general, smoothing treatment which approximates a broken line obtained by connecting a plurality of machining points by a curve so as to generate a smooth tool path is known. If such smoothing treatment is carried out across-the-board at all of the machining path including parts with shapes of large bend angles, the error in shape from the desired machined shape increases and thus, the machined shape may be conversely degraded. Considering this point, for example, Patent Literature 1 describes a method of comparing a bend angle of a broken line with a predetermined threshold value and validating (turning on) or invalidating (turning off) the smoothing treatment in accordance with their relative magnitudes.
In this regard, machining point data is generally prepared by utilizing a CAD/CAM system etc. For this reason, even if a curvature of a machined surface is constant, the lengths of the broken line segments (block lengths) vary and the bend angles also vary. Therefore, if the method, like the one described in the above Patent Literature 1, of turning the smoothing treatment on and off at a specific bend angle is used, there will be portions where the smoothing treatment is turned on and portions where it is turned off regardless of the curvature of the machined surface being constant and therefore, it is difficult to obtain a smooth machined surface.